You can rent an electric snake, but the ones I have seen have been so abused that it is dangerous for you to use them. A plumber will unplug it and may also be able to tell if there is any other problem with the system.

: I have a unique problem with my kitchen sink draining slowly.: I took the trap off under the sink and it is fine, very clean. I went into the basement and opened the cast iron drain pipe and found the problem, I think. When I had the kitchen sink water turned on, I watched the water come down into the cellar drain pipe and flow under the basement floor. After a little while, the water started backing up. I imagine that it is backing up all the way upstairs into the kitchen sink. It happens only after the water has been running for a while. I tried snaking it with a 30 foot snake, it went in the pipe all the way, but it didn't seem to help. The hook on the end of it came out with fishy smelling black stuff, so I guess it is connected to the sewer? The house was built in 1950. What can I do to fix it, the most inexpensive way? Is there a chemical we can put directly into the basement drain? Or should I rent and try one of those electric snakes? Or, God forbid, gulp, call a plumber? Thanks for your help!